Correct. In the Web, it's considered a Calculated Field (Settings -> Fields -> Calculated Fields) as is typically applied to a sourcetype. And on the backend that is just written to props.conf
[your_sourcetype]
EVAL-my_val = case(match(_raw,"aaa\s+:\d\d:"),2,match(_raw,"aaa\s+:\d:"),1,true(),"n/a")
Not sure what your environment looks like, but just note that this is considered a knowledge object and as such has permissions (who can read/write it) and scope (private/app/global) associated with it.
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